An orthogonal frequency division multiplex (ODFM) signal may include scattered pilots on every nth carrier, which may be used to estimate channel frequency responses at scattered pilot carrier frequencies. Where channel frequency responses are only available for every nth carrier frequency, a corresponding channel impulse response may exhibit cyclic ambiguity, which may impact symbol timing recovery and channel equalization.
Cyclic ambiguity may be avoided by constraining echoes to lie within a time interval of Tu/2n of the strongest path, where Tu represents an effective symbol duration, and centering the channel impulse response at the strongest component. Such an approach may be sufficient for small to medium single frequency networks (SFNs), and non-SFN environments. For example, for 8K OFDM and n=3 in an 8 MHz channel of a relatively small SFN, Tu may be approximately 896 micro-seconds (μs), and it may be reasonable to assume that all pre and post echoes are within 150 μs (i.e. Tu/6) of a main path.
In larger SFN environments, post echo delays may exceed Tu/2n. If post-echoes are constrained to be within Tu/2n of a main path, a longer delay post-echo may appear as a pre-echo of a subsequent window, referred to herein as wrap-around.
For larger SFN environments, an estimated channel impulse response may be iteratively adjusted based on signal-to-noise ratios (SNRs) to identify an optimum or suitable placement. Trial and error placement at periodic intervals may, however, lead to loss of data.
In the drawings, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears.